coatings

Review

Hybrid Sol-Gel Coatings: Smart and Green Materialsfor Corrosion Mitigation

Rita B. Figueira 1,*, Isabel R. Fontinha 1, Carlos J. R. Silva 2 and Elsa V. Pereira 1

1 LNEC, Laboratório Nacional de Engenharia Civil, Av. Brasil 101, Lisboa 1700-066, Portugal;rfontinha@lnec.pt (I.R.F.); epereira@lnec.pt (E.V.P.)

2 Centro de Química, Universidade do Minho, Campus de Gualtar, Braga 4710-057, Portugal;csilva@quimica.uminho.pt

* Correspondence: rmfigueira@lnec.pt or rita@figueira.pt; Tel.: +351-218-443-634 or +351-966-430-142

Academic Editor: Naba K. DuttaReceived: 17 December 2015; Accepted: 11 March 2016; Published: 16 March 2016

Abstract: Corrosion degradation of materials and metallic structures is one of the major issuesthat give rise to depreciation of assets, causing great financial outlays in their recovery and orprevention. Therefore, the development of active corrosion protection systems for metallic substratesis an issue of prime importance. The promising properties and wide application range of hybridsol-gel-derived polymers have attracted significant attention over recent decades. The combination oforganic polymers and inorganic materials in a single phase provides exceptional possibilities to tailorelectrical, optical, anticorrosive, and mechanical properties for diverse applications. This unlimiteddesign concept has led to the development of hybrid coatings for several applications, such astransparent plastics, glasses, and metals to prevent these substrates from permeation, mechanicalabrasion, and corrosion, or even for decorative functions. Nevertheless, the development of newhybrid products requires a basic understanding of the fundamental chemistry, as well as of theparameters that influence the processing techniques, which will briefly be discussed. Additionally,this review will also summarize and discuss the most promising sol-gel coatings for corrosionprotection of steel, aluminium, and their alloys conducted at an academic level.

Keywords: hybrid; coatings; sol-gel; corrosion; steel; green materials; aluminium; metallic

1. Introduction

The major degradation mechanism of metallic materials is corrosion (Figure 1). Corrosion canbe generally defined as the reaction of a metallic material with its environment and the products ofthis reaction may be solid, liquid, or gaseous [1–4]. Both the physical and chemical properties of theproducts are important since they may influence the subsequent rate of reaction. In the case of steel,Cl´, O2, and H2O species, as well as electron transport, all play extremely important roles in thecorrosion process [5–9].

 

Coatings 2016, 6, 12; doi:10.3390/coatings6010012  www.mdpi.com/journal/coatings 

Review 

Hybrid Sol‐Gel Coatings: Smart and Green Materials for Corrosion Mitigation 

Rita B. Figueira 1,*, Isabel R. Fontinha 1, Carlos J. R. Silva 2 and Elsa V. Pereira 1 

1  LNEC, Laboratório Nacional de Engenharia Civil, Av. Brasil 101, Lisboa 1700‐066, Portugal; 

rfontinha@lnec.pt (I.R.F.); epereira@lnec.pt (E.V.P.) 2  Centro de Química, Universidade do Minho, Campus de Gualtar, Braga 4710‐057, Portugal; 

csilva@quimica.uminho.pt 

*  Correspondence: rmfigueira@lnec.pt or rita@figueira.pt; Tel.: +351‐218‐443‐634 or +351‐966‐430‐142 

Academic Editor: Naba K. Dutta 

Received: 17 December 2015; Accepted: 11 March 2016; Published: March 2016 

Abstract: Corrosion degradation of materials and metallic structures is one of the major issues that 

give  rise  to  depreciation  of  assets,  causing  great  financial  outlays  in  their  recovery  and  or 

prevention.  Therefore,  the  development  of  active  corrosion  protection  systems  for  metallic 

substrates is an issue of prime importance. The promising properties and wide application range of 

hybrid  sol‐gel‐derived  polymers  have  attracted  significant  attention  over  recent  decades.  The 

combination of organic polymers and  inorganic materials  in a single phase provides exceptional 

possibilities  to  tailor  electrical,  optical,  anticorrosive,  and  mechanical  properties  for  diverse 

applications. This unlimited design  concept  has  led  to  the development  of  hybrid  coatings  for 

several applications, such as  transparent plastics, glasses, and metals  to prevent  these substrates 

from  permeation,  mechanical  abrasion,  and  corrosion,  or  even  for  decorative  functions. 

Nevertheless,  the  development  of  new  hybrid  products  requires  a  basic  understanding  of  the 

fundamental chemistry, as well as of the parameters that influence the processing techniques, which 

will  briefly  be  discussed. Additionally,  this  review will  also  summarize  and  discuss  the most 

promising sol‐gel coatings for corrosion protection of steel, aluminium, and their alloys conducted 

at an academic level. 

Keywords: hybrid; coatings; sol‐gel; corrosion; steel; green materials; aluminium; metallic 

 

1. Introduction 

The major degradation mechanism of metallic materials is corrosion (Figure 1). Corrosion can 

be generally defined as the reaction of a metallic material with its environment and the products of 

this reaction may be solid, liquid, or gaseous [1–4]. Both the physical and chemical properties of the 

products are important since they may influence the subsequent rate of reaction. In the case of steel, 

Cl−, O2, and H2O  species, as well as  electron  transport,  all play  extremely  important  roles  in  the 

corrosion process [5–9]. 

 

Figure 1. Corrosion of steel in a bridge: (a) general view; (b) detail of the thickness of the corrosion 

layer formed. Figure 1. Corrosion of steel in a bridge: (a) general view; (b) detail of the thickness of the corrosionlayer formed.

Coatings 2016, 6, 12; doi:10.3390/coatings6010012 www.mdpi.com/journal/coatings

Coatings 2016, 6, 12 2 of 19

Corrosion prevention in a specific environment is usually performed using a corrosion resistantmaterial, or at least one with exceptional lifetime. There are several methods for protection againstcorrosion that are based on electrochemical principles [9]. Alternatively, some of the methods adoptthe obvious task of separating the metal from the environment. The success of this depends uponthe chemical or electrochemical resistance of the protective layer. In particular situations it may bepossible to adjust the environment in order to turn it less corrosive. Generally, corrosion preventiontechnology uses one or more of the following methods [2,5,7,8,10,11]:

1. Adopting metals with elements that enrich the surface with a corrosion-resistant componentduring the corrosion process;

2. Addition of aqueous inhibitors, which adsorb strongly on the metal surface and prevent thereaction with the oxidizing agent; and

3. Deposition of protective coatings.

The choice of the most effective method is not always simple and easy. This choice is partlygoverned by the actual environmental conditions and partly by economic considerations. The latterinclude not only the initial cost of application but also the replacement of corroded parts and, insome cases, renewal of the protecting medium. These are frequently the determining factors andthe practicing corrosion engineer will use economic parameters as familiarly as scientific data indetermining their choice (Figures 2 and 3).

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Corrosion prevention in a specific environment is usually performed using a corrosion resistant 

material, or at least one with exceptional lifetime. There are several methods for protection against 

corrosion that are based on electrochemical principles [9]. Alternatively, some of the methods adopt 

the obvious task of separating the metal from the environment. The success of this depends upon the 

chemical  or  electrochemical  resistance  of  the  protective  layer.  In  particular  situations  it may  be 

possible to adjust the environment in order to turn it less corrosive. Generally, corrosion prevention 

technology uses one or more of the following methods [2,5,7,8,10,11]: 

1. Adopting metals with elements  that enrich  the surface with a corrosion‐resistant component 

during the corrosion process; 

2. Addition of aqueous  inhibitors, which adsorb strongly on  the metal surface and prevent  the 

reaction with the oxidizing agent; and 

3. Deposition of protective coatings. 

The choice of  the most effective method  is not always simple and easy. This choice  is partly 

governed by the actual environmental conditions and partly by economic considerations. The latter 

include not only the initial cost of application but also the replacement of corroded parts and, in some 

cases,  renewal  of  the  protecting medium.  These  are  frequently  the  determining  factors  and  the 

practicing  corrosion  engineer  will  use  economic  parameters  as  familiarly  as  scientific  data  in 

determining their choice (Figures 2 and 3). 

 

Figure 2. (a) In situ studies of coatings for steel protection in a marine environment; (b,c) anodized 

aluminium specimens after exposure in a natural industrial environment. 

 

Figure 3. Aluminium components in a building façade. 

Figure 2. (a) In situ studies of coatings for steel protection in a marine environment; (b,c) anodizedaluminium specimens after exposure in a natural industrial environment.

Coatings 2016, 6, 12  2 of 19 

Corrosion prevention in a specific environment is usually performed using a corrosion resistant 

material, or at least one with exceptional lifetime. There are several methods for protection against 

corrosion that are based on electrochemical principles [9]. Alternatively, some of the methods adopt 

the obvious task of separating the metal from the environment. The success of this depends upon the 

chemical  or  electrochemical  resistance  of  the  protective  layer.  In  particular  situations  it may  be 

possible to adjust the environment in order to turn it less corrosive. Generally, corrosion prevention 

technology uses one or more of the following methods [2,5,7,8,10,11]: 

1. Adopting metals with elements  that enrich  the surface with a corrosion‐resistant component 

during the corrosion process; 

2. Addition of aqueous  inhibitors, which adsorb strongly on  the metal surface and prevent  the 

reaction with the oxidizing agent; and 

3. Deposition of protective coatings. 

The choice of  the most effective method  is not always simple and easy. This choice  is partly 

governed by the actual environmental conditions and partly by economic considerations. The latter 

include not only the initial cost of application but also the replacement of corroded parts and, in some 

cases,  renewal  of  the  protecting medium.  These  are  frequently  the  determining  factors  and  the 

practicing  corrosion  engineer  will  use  economic  parameters  as  familiarly  as  scientific  data  in 

determining their choice (Figures 2 and 3). 

 

Figure 2. (a) In situ studies of coatings for steel protection in a marine environment; (b,c) anodized 

aluminium specimens after exposure in a natural industrial environment. 

 

Figure 3. Aluminium components in a building façade. Figure 3. Aluminium components in a building façade.

Carbon steel is used in large amounts in the construction industry. By their nature of limited alloycontent, usually less than 2% carbon of total weight, carbon steels are susceptible to very high corrosion

Coatings 2016, 6, 12 3 of 19

rates in aggressive solutions and atmospheres [11,12]. The traditional surface passivation treatment forsteel is obtained by conversion coating, such as phosphating and chromating, which produces a layerof corrosion-protective products capable of resisting further chemical attack. Phosphate conversioncoatings (phosphating) have found use in the passivation of steel surfaces. Phosphate films havea good adherence, are somewhat porous and, therefore, form an excellent basis for paints, lacquers, andrust-preventing oils and greases. The protective ability of the phosphate coating alone is insufficient,as phosphate films are formed at cathodic sites. Small, randomly-spaced anodic sites remain uncoatedon the metal surface, resulting in the formation of pinholes. The presence of pinholes in phosphateconversion layers results in rapid film degradation under corrosive conditions, such as salt watercontact. Chromate conversion coating (chromating) is a process resembling phosphating and often usedas a passivating treatment after phosphating, but it is not used alone on steel. Chromating processesare rapid, cheap, and easily carried out. They all involve an oxidizing treatment of the metal surfacein a bath containing chromic acid or chromates and accelerating anions. Formed layers containingchromates and chromium oxide are usually very thin, very dense and have a gel structure. Adherence isvery good and the films form an excellent basis for painting [13–15].

Aluminium alloys are extensively used in the construction industry, mainly in buildingcomponents (Figure 3). Chromate and chromate-phosphate conversion coatings have also beenwidely used for several decades to provide improvement in corrosion resistance and paint adhesion onaluminium and its alloys, namely, of 5000 and 6000 series (for extruded products), and of 3000 series(for sheet products), which are used for different applications in building and construction.

The protection of metallic substrates with coatings has been an active area of materials science formany years [12–15]. Moreover, over the decades, changing regulation governing both environmentalissues and human health concerns imposed severe restrictions on the use of hazardous compoundssuch as VOCs (Volatile Organic Compounds), HAPs (Hazardous Air Pollutants), and use of Cr(VI).Hexavalent chromium-containing compounds used in chromate conversion coatings are knownto be toxic and carcinogenic; consequently, recent legislation imposes strong limitations in futureuse of chromium compounds yielding to a great effort in developing chromate-free coatings.These restrictions, and the lack of materials capable of accomplishing all technological needs, naturally,led to the development of new corrosion protective coating systems. Intense research efforts for thedevelopment of new smart and green systems for corrosion protection have been carried out in the lastfew decades. The paint industry is environmentally driven to pursue the reduction of the use of VOCs,such as solvents, resulting in an enormous attempt to find water-based alternatives. The developmentsof low-VOC, low-HAP, and non-toxic surface treatments, capable of providing effective corrosionprotection, have been being chased and the sol-gel methodology is considered an adequate alternativeto produce smart and green coatings.

The challenges for developing new coatings come from many areas of materials science.Metallurgy and electrochemistry are required in order to understand the alloy microstructure andcorrosion process. Surface inorganic and polymer chemistries are needed for the design of the coatingsystem and physics for the transport properties within the coating system.

This review presents a summary of the main research achievements in the development, usingthe sol-gel method, of hybrid sol-gel coatings for protection of steel and aluminium substrates againstcorrosion. A rational selection of the most relevant contributions was made in order to providea reliable and a trustworthy analysis of the present consolidated know-how and perspectives to furtherdevelopment in this area. A comparative analysis between the potentialities of hybrid sol-gel coatingswith the existing methods currently available in the mitigation of corrosion is also performed.

2. General Background of Sol-Gel Coatings for Corrosion Mitigation

The sol-gel process is a chemical synthesis method where an alkoxide network is created byprogressive condensation reactions of molecular precursors from a liquid medium [16–18]. The processwas initially used for the preparation of inorganic materials, such as glasses and ceramic. The sol-gel

Coatings 2016, 6, 12 4 of 19

process to produce hybrid coatings, compared to other methods, has several advantages. This process isoften classified as a “green” technology since it uses compounds that do not introduce impurities into theend product (e.g., ceramic processing), is waste-free, excludes the stage of washing, and the processingtemperature is generally low, frequently close to room temperature [19]. Additionally, materials withhigh specific porosity and specific surface area can be easily obtained by this method, which alsoallows the incorporation of substances, such as inhibitors. The low synthesis temperatures minimizethe thermal volatilization and degradation of the entrapped species [16,18]. Additionally, the organiccomponent, other additives, such as Ce(III), Ce(IV) or Cr(III) could be easily incorporated into thesol-gel system, increasing the corrosion resistance of the metal substrates. In addition to the corrosioninhibitors, sacrificial metal pigments, such as zinc, aluminium, magnesium, and their alloy particles,can also be included into the sol-gel coating formula according to the different metal substrates to beprotected. As chemical precursors are predominantly in a liquid form, it is possible to cast coatings incomplex shapes and to produce thin films without the need for machining or melting. There are severaltechniques for the deposition of coatings on metals, including physical vapor deposition, chemicalvapor deposition, electrochemical deposition, plasma spraying, and the sol-gel process.

The first step to implement a synthesis through the sol-gel method is to select the chemicalcompounds, known as the precursors (Figures 4 and 5) that are combined forming a sol. This termdesignates a stable suspension of colloidal particles within a liquid [16,17]. This first step is a typicalchemical transformation aiming the formation of a sol of colloidal particles or a solution of oligomers(small polymers). As sol is a fluid, it can be cast in a mold, or be applied on a surface using variousshaping techniques, e.g., spraying on a surface, dipping, or spinning through a set of rotating nozzles(Figure 4). As the sol is a relatively stable solution it can be stored for a certain time before furthercasting, or applied by any of the above referred methods. Spraying and electrodeposition processesemerged recently and could be the major sol-gel coating application methods in the near future.

To achieve gelation, the chemical transformations of the sol in gel must be allowed to proceeduntil a single and interconnected network forms, only limited by the walls of the container and themass of the reactive mixture. Another step in sol-gel art is drying, which is a very critical step. In manyinstances, a dry gel needs further thermal treatment to be densified [20]. The sol-gel process versatilityallows incorporation of additional components that introduce material’s complementary functionalities,such as UV protection [21,22], anti-fouling [23,24], anti-reflection [25–29], moisture resistance [29–33],corrosion and adhesion protection [34–53], along with the enhancement of mechanical, thermal, andoptical properties, opening a wide range of application in several fields of science. Furthermore, thismethod allows the control of several experimental conditions leading to a simple sequence that maybe tuned to the nature, final shape, and desired function.

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process was initially used for the preparation of inorganic materials, such as glasses and ceramic. The 

sol‐gel process to produce hybrid coatings, compared to other methods, has several advantages. This 

process  is often classified as a “green”  technology since  it uses compounds  that do not  introduce 

impurities  into  the  end  product  (e.g.,  ceramic  processing),  is waste‐free,  excludes  the  stage  of 

washing, and the processing temperature is generally low, frequently close to room temperature [19]. 

Additionally, materials with high specific porosity and specific surface area can be easily obtained 

by  this method, which  also  allows  the  incorporation  of  substances,  such  as  inhibitors.  The  low 

synthesis temperatures minimize the thermal volatilization and degradation of the entrapped species 

[16,18]. Additionally, the organic component, other additives, such as Ce(III), Ce(IV) or Cr(III) could 

be  easily  incorporated  into  the  sol‐gel  system,  increasing  the  corrosion  resistance  of  the metal 

substrates. In addition to the corrosion inhibitors, sacrificial metal pigments, such as zinc, aluminium, 

magnesium, and their alloy particles, can also be included into the sol‐gel coating formula according 

to the different metal substrates to be protected. As chemical precursors are predominantly in a liquid 

form, it is possible to cast coatings in complex shapes and to produce thin films without the need for 

machining or melting. There are several techniques for the deposition of coatings on metals, including 

physical vapor deposition, chemical vapor deposition, electrochemical deposition, plasma spraying, 

and the sol‐gel process. 

The  first  step  to  implement a  synthesis  through  the  sol‐gel method  is  to  select  the  chemical 

compounds, known as the precursors (Figures 4 and 5) that are combined forming a sol. This term 

designates a stable suspension of colloidal particles within a liquid [16,17]. This first step is a typical 

chemical transformation aiming the formation of a sol of colloidal particles or a solution of oligomers 

(small polymers). As sol is a fluid, it can be cast in a mold, or be applied on a surface using various 

shaping techniques, e.g., spraying on a surface, dipping, or spinning through a set of rotating nozzles 

(Figure 4). As the sol is a relatively stable solution it can be stored for a certain time before further 

casting, or applied by any of the above referred methods. Spraying and electrodeposition processes 

emerged recently and could be the major sol‐gel coating application methods in the near future. 

To achieve gelation, the chemical transformations of the sol in gel must be allowed to proceed 

until a single and interconnected network forms, only limited by the walls of the container and the 

mass of the reactive mixture. Another step  in sol‐gel art  is drying, which is a very critical step. In 

many instances, a dry gel needs further thermal treatment to be densified [20]. The sol‐gel process 

versatility allows incorporation of additional components that introduce material’s complementary 

functionalities, such as UV protection [21,22], anti‐fouling [23,24], anti‐reflection  [25–29], moisture 

resistance  [29–33],  corrosion  and  adhesion  protection  [34–53],  along  with  the  enhancement  of 

mechanical, thermal, and optical properties, opening a wide range of application in several fields of 

science. Furthermore, this method allows the control of several experimental conditions leading to a 

simple sequence that may be tuned to the nature, final shape, and desired function. 

 

Figure 4. Examples of processing routes to obtain sol‐gel coatings. Adapted from [54]. Figure 4. Examples of processing routes to obtain sol-gel coatings. Adapted from [54].

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Figure 5. Examples of precursors used in the synthesis of hybrid materials. 

There are two main methods to prepare sol‐gel coatings: the inorganic method and the organic 

one.  The  first method  involves  the  evolution  of  networks  through  the  formation  of  a  colloidal 

suspension (habitually, oxides) and gelation of the sol (colloidal suspension of very small particles 

that may vary  from 1  to 100 nm)  to  form a network  in a continuous  liquid phase  [16]. The most 

commonly used method to prepare sol‐gel coatings is the organic one, which normally starts with a 

solution of monomeric metal or metalloid alkoxide precursors M(OR)n in an alcohol or other  low‐

molecular weight organic solvent. M represents a network‐forming element, such as Si, Ti, Zr, Al, Fe, 

B, etc., and R is typically an alkyl group (CxH2x+1). This later approach has the advantage of obtaining 

organic‐inorganic hybrid materials combining  the behavior of  the organic component  (polymeric) 

with the stiffness of the inorganic (alkoxide) backbone [55]. 

Generally, the sol‐gel method used to obtain hybrid materials (organic method) occurs in four 

stages:  (i) a hydrolysis  reaction of  the metal alkoxide groups  (–OR) are  replaced by  the hydroxyl 

groups (–OH) (Equation (1)); (ii) condensation and polymerization of monomers to form chains and 

particles; (iii) growth of the particles; and (iv) agglomeration of the polymer structures followed by 

the formation of networks that extend throughout the liquid medium resulting in a gel. Indeed, both 

the hydrolysis and condensation reactions occur at the same time once the hydrolysis reaction has 

been  initiated.  Both  the  hydrolysis  and  condensation  steps  produce  low molecular weight  by‐

products, such as alcohol and water. 

M(OR)z + H2O  M(OH)(OR)z−1 + R(OH)  (1) 

The mechanism  of  Equation  (1)  involves  the  nucleophilic  attack  of  the  negatively‐charged 

hydroxyl group (HOδ−) to the positively‐charged metallic atom (Mδ+). Hydroxyl groups are produced 

followed by the transference of one proton (H+) and breaking of the M–OR bond and formation of an 

alcohol R–OH  [16]. This process  repeats  successively  until  all  the  alkoxide  groups  are  replaced. 

Subsequently, the condensation reactions of the resulting hydroxyl groups takes place, in which the 

hydrolyzed  species M(OR)x(OH)y  (x = 0, 1,…, z−1; y = z−x) give birth  to  the  formation of a  three‐

dimensional network  [16]. The condensation process may occur via oxolation or olation reactions 

[16]. In the oxolation two hydrolyzed species react forming “oxo” bonds M–O–M (Equations (2) and 

(3)) and in olation (Equation (4)) one of those species is protonated, generating M–OH–M bonds [16]. 

Oxalation: 

(OR)x(OH)y−1 M–OH + HO–M(OR)x(OH)y−1  (OR)x(OH)y−1 M–O–M(OR)x(OH)y−1 + H2O  (2) 

(OR)x(OH)y−1 M–OH + RO–M(OR)x−1(OH)y (OR)x(OH)y−1 M–O–M(OR)x−1(OH)y + R–OH (3) 

Figure 5. Examples of precursors used in the synthesis of hybrid materials.

There are two main methods to prepare sol-gel coatings: the inorganic method and the organic one.The first method involves the evolution of networks through the formation of a colloidal suspension(habitually, oxides) and gelation of the sol (colloidal suspension of very small particles that mayvary from 1 to 100 nm) to form a network in a continuous liquid phase [16]. The most commonlyused method to prepare sol-gel coatings is the organic one, which normally starts with a solutionof monomeric metal or metalloid alkoxide precursors M(OR)n in an alcohol or other low-molecularweight organic solvent. M represents a network-forming element, such as Si, Ti, Zr, Al, Fe, B, etc.,and R is typically an alkyl group (CxH2x+1). This later approach has the advantage of obtainingorganic-inorganic hybrid materials combining the behavior of the organic component (polymeric) withthe stiffness of the inorganic (alkoxide) backbone [55].

Generally, the sol-gel method used to obtain hybrid materials (organic method) occurs in fourstages: (i) a hydrolysis reaction of the metal alkoxide groups (–OR) are replaced by the hydroxylgroups (–OH) (Equation (1)); (ii) condensation and polymerization of monomers to form chains andparticles; (iii) growth of the particles; and (iv) agglomeration of the polymer structures followed bythe formation of networks that extend throughout the liquid medium resulting in a gel. Indeed, boththe hydrolysis and condensation reactions occur at the same time once the hydrolysis reaction hasbeen initiated. Both the hydrolysis and condensation steps produce low molecular weight by-products,such as alcohol and water.

MpORqz ` H2O ÑÑÑ MpOHqpORqz´1 ` RpOHq (1)

The mechanism of Equation (1) involves the nucleophilic attack of the negatively-chargedhydroxyl group (HOδ´) to the positively-charged metallic atom (Mδ`). Hydroxyl groups areproduced followed by the transference of one proton (H+) and breaking of the M–OR bond andformation of an alcohol R–OH [16]. This process repeats successively until all the alkoxide groupsare replaced. Subsequently, the condensation reactions of the resulting hydroxyl groups takesplace, in which the hydrolyzed species M(OR)x(OH)y (x = 0, 1, . . . , z´1; y = z´x) give birth tothe formation of a three-dimensional network [16]. The condensation process may occur via oxolationor olation reactions [16]. In the oxolation two hydrolyzed species react forming “oxo” bonds M–O–M(Equations (2) and (3)) and in olation (Equation (4)) one of those species is protonated, generatingM–OH–M bonds [16].

Oxalation:

pORqxpOHqy´1 M–OH ` HO–MpORqxpOHqy´1 ÑÑÑ pORqxpOHqy´1 M–O–MpORqxpOHqy´1 ` H2O (2)

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pORqxpOHqy´1 M–OH ` RO–MpORqx´1pOHqy ÑÑÑ pORqxpOHqy´1 M–O–MpORqx´1pOHqy ` R–OH (3)

Olation:

pORqxpOHqy´1 M–OH ` H2O–MpORqxpOHqy´1 ÑÑÑ pORqxpOHqy´1 M–OH–MpORqxpOHqy´1 ` H2O (4)

Upon drying, these small molecules are driven off and the network shrinks as further condensationmay occur. These processes are fundamentally affected by the initial reaction conditions, such astemperature, pH, molar ratios of reactants, and solvent composition, among others. Cracks are easy toform if the film formation conditions are not carefully controlled [16,55].

The production process of so-called low-bulk density silica involving the hydrolysis oftetraethoxysilane (TEOS) (Figure 5) in the presence of cationic surfactants was patented in 1971,and the organic-inorganic hybrid materials prepared by the sol-gel process started in the middleof the 1980s. Several inorganic coatings have been studied for corrosion protection, specificallyoxide coatings obtained by sol-gel processing. In spite of all the advantages of sol-gel processing,sol-gel oxide coatings suffer from several disadvantages. Generally, they are highly porous with lowmechanical integrity, so annealing or sintering at high temperatures is required in order to achievea dense microstructure. Consequently, sintering at high temperatures might introduce cracks and/ordelamination into the sol-gel coatings due to a large mismatch of thermal expansion coefficientsand possible chemical reactions at the interface. High temperatures also limit the application ofsol-gel coatings on temperature-sensitive substrates and devices. One viable approach to dense,sol-gel-derived coatings without post-deposition annealing at elevated temperatures is to synthesizehybrid coatings. When appropriate chemical composition and processing conditions are applied,relatively dense and non-porous hybrid coatings can be developed. These materials can combinedensity, flexibility, hardness, and easy processing of the organic component with the hardness, chemical,and weather resistance of the inorganic component, thereby exhibiting multifunctional behavior.

In the first attempt to implement a classification of hybrid materials obtained by sol-gel, the hybridnetworks were divided into two general classes: class I and class II. This classification was establishedconsidering the interaction between the inorganic and organic components and the first detailed reporton this topic was made in 1994 [56]. The initial work concerning corrosion protection offered by sol-gelcoatings was conducted using class I hybrids, in which the inorganic and organic components arebonded by van der Waals forces. Messadeq et al. [57] proved that the corrosion resistance of stainlesssteel (SS) could be increased by a factor of 30 using hybrid coatings (class I) prepared by the dispersionof poly(methyl methacrylate), into a zirconia sol. Nevertheless, phase separation and subsequentdelamination caused long-term failure of the film. The use of class II hybrids, in which the organic andinorganic components are linked by covalent bonds, could reduce the phase separation and delaminationas well as increase resistance. Thereby, the research in this area headed to organo-functional alkoxysilaneswhich have been extensively used in the last decade to prepare hybrid coatings. Mainly due to theiradhesive and coupling nature with other molecules and substrates, and by the increased resistanceobtained with these materials. The studies concerning protection of metals using siloxane-based filmshave consistently demonstrated that siloxanes, a group of environmentally-compliant chemicals, couldefficiently protect metals against different forms of corrosion. A large variety of chemical species couldbe incorporated and the wide range of available chemical precursors for the sol-gel process wouldallow the production of a diversity of economical hybrid coating materials at mild synthesis conditions.Furthermore, hybrid materials could be produced by low-cost and high-efficiency common spray, spin,and dip-coating methods (Figure 4), which enhances the production of layers with adjusted chemicaland physical properties. These materials can be potentially used as eco-friendly corrosion protectionsystems providing an improved level of performance compared to the traditional organic coatingsystems showing superior weatherability. Several review papers about the progress and developmentof sol-gel coatings on metal substrates have been published. The most significant contribution wasgiven by Wang and Bierwagen in 2009 [55], where the progress of sol-gel protective coatings was

Coatings 2016, 6, 12 7 of 19

discussed and the most significant advances until 2007 were reported. Later in 2012, Balgude andSabnis [19] discussed the potential of hybrid sol-gel coatings for the production of multifunctionalpre-treatments for metallic substrates. One year later, Abdolah et al. [34] discussed the most relevantworks published on sol-gel coatings with self-healing properties. In 2014 Figueira et al. [58] publisheda review of the most recent and relevant achievements on the development of hybrid sol-gel coatingsfor corrosion mitigation of metallic substrates. The general consensus of these studies revealed thatthese materials have a good corrosion resistance and could be effective alternatives to the traditionalorganic-based coating systems.

2.1. Corrosion Protective Sol-Gel Coatings in Steel Substrates

Steel is a common and versatile substrate with several applications. By definition, steel mustbe at least 50% (w/w) iron and must contain one or more alloying element. These elements includecarbon, manganese, silicon, nickel, chromium, molybdenum, vanadium, titanium, niobium, andaluminum [12]. Generally, these substrates are passivated by the use of chemical conversion layers(chromating and phosphating). Several studies were found on developing hybrid sol-gel coatingsfor application on iron-based alloy surfaces. Nevertheless, only the most representative publishedin the last decade, will be referred. Several iron-based substrates have been studied, such as carbonsteel [46,59–63], 304, and 316 stainless steel [47,64–69]. Chou et al. [70] in 2003 produced hybridcoatings by copolymerizing TEOS and 3-methacryloxypropyltrimethoxysilane (MAPTS) (Figure 5)in a two-step acid-catalyzed process. The obtained coatings were uniform, defect-free with excellentadhesion and flexibility that enhanced the corrosion protection of 304 and 316 stainless steel substrates.Gunji et al. prepared polysilsesquioxanes through acid-catalyzed hydrolytic polycondensation ofmethyltriethoxysilane (MTES) (Figure 5) and tri-isopropoxy(methyl)silane [71]. Castro et al. obtainedby dipping and by electrophoretic deposition (EPD) homogeneous and crack-free coatings preparedunder basic catalysis of TEOS and MTES. The EPD method produced thicker coatings with enhancedcorrosion resistance of the AISI 304 stainless steel [72]. Recently sol-gel coatings synthesized from MTESand TEOS for stainless steel AISI 316-L with different MTES:TEOS molar ratios were also reported [73].Jianguo et al. studied a hybrid SiO2 gel/Dacromet composite coating under acid-catalyzed hydrolysiswith TEOS and MAPTS in ethanol. This system enhanced the erosion-corrosion resistance of Dacrometmarkedly in the carbon steel [74].

Later, Wang and Akid showed that a multi-layer hydrophobic coating with a doped ceriumnitrate/hydrophobic layer provided good corrosion resistance to mild steel [75]. Quian et al. tested hybridcoatings cured with pre-hydrolyzed amino-siloxane on carbon steel, which exhibited good corrosionresistance against alkaline, acidic and saline conditions [62]. Kannan et al. [76] produced coatingsin a two-step reaction process by copolymerizing 2-methacryloyloxyethyl-phosphate containinga polymerizable methacrylate group and a functional group with MAPTS. It was concluded thatthis hybrid gel coatings improved corrosion resistance of mild steel in comparison with the uncoatedsamples. The presence of strong interfacial interaction was attributed to the interaction of phosphategroup with the metallic substrate. Guin et al. [77] prepared hybrid nanotitania-silica compositecoating from polycondensation of titanium-isopropoxide and N-phenyl-3-aminopropyltriethoxysilane.These coatings offered hydrophobic surface with good corrosion protection to steel sheets.Recent studies have reported the use of aqueous hybrid silica coatings on mild steel using GPTMS andaminopropyltriethoxysilane (APTES) sols [78] and TEOS and GPTMS together with various quantitiesof colloidal silica suspension [60]. Sol-gel coatings prepared from condensation and polymerization ofTEOS and MAPTS, TEOS and MTES, TMOS and MAPTS, or TMOS and MTES mixtures in three molarratios were also reported by Criado et al. [79,80]. For carbon steel, hybrid sol-gel coatings preparedfrom 3-(trimethoxysilyl) propyl methacrylate and bis [2-(methacryloyloxy) ethyl] phosphate via thesol-gel process [81] were applied. Generally, the recent publications, from 2013 to 2015 are followinga tendency found previously [58], where the majority of the reported papers on this substrates useTEOS; GPTMS, MAPTS, and MTES.

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2.2. Corrosion Protective Sol-Gel Coatings in Aluminium Substrates

Aluminum has a wide range of applications particularly in construction, transport, packaging,and engineering (Figure 6). However, it is in the transport sector, particularly the aviation industry,that these substrates find their main uses [82]. This may explain the strong research efforts on searchingfor efficient coatings and pre-treatments to replace the use of hexavalent chromium-based surfacetreatments [58].

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2.2. Corrosion Protective Sol‐Gel Coatings in Aluminium Substrates 

Aluminum has a wide range of applications particularly in construction, transport, packaging, 

and engineering (Figure 6). However, it is in the transport sector, particularly the aviation industry, 

that  these  substrates  find  their main  uses  [82].  This may  explain  the  strong  research  efforts  on 

searching for efficient coatings and pre‐treatments to replace the use of hexavalent chromium‐based 

surface treatments [58]. 

 

Figure 6. Main end‐uses for aluminum products in Europe (data from 2013). Adapted from [82]. 

Sol‐gel coatings have been extensively studied as a promising environmental friendly alternative 

for replacement of hexavalent chromium‐based surface treatments of aluminium alloys [58]. This is 

not an easy task since these conversion coatings are very effective in promoting both adhesion and 

corrosion protection due to the active corrosion‐inhibiting effect of Cr(VI) ions present in the film, 

besides the physical barrier induced by the film itself. 

Solely inorganic sol‐gel coating systems are porous and require densification with high curing 

temperatures  (above  300  °C) which  can  induce  sensitisation  as well  as  the  loss  of mechanical 

properties of the aluminium alloy substrate [83], making them unsuitable for corrosion protection of 

these alloys. However, hybrid sol‐gel coatings, due to the presence of the organic component, which 

increases flexibility and density, require much lower curing temperatures (below 150 °C) to achieve 

the desired barrier properties making them more interesting and suitable for aluminium corrosion 

protection (Figure 7). Additionally, they have the ability to incorporate additives, such as inhibitors 

and nanoparticles, etc., which further enhance corrosion and mechanical resistance of these types of 

coatings [55,84]. 

In 2001, Parkhill et al. [85] prepared epoxy‐silica sol‐gel films on 2014‐T3 aluminium alloy. These 

results showed excellent barrier properties and enhanced corrosion resistance  in salt spray  test  in 

comparison to the chromate conversion pre‐treatment conventionally applied to aircraft alloys. From 

this point forward, several types of organically‐modified sol‐gel coatings have been studied for the 

corrosion protection of aluminium alloys  [86–94]. Most of  them are based on organo‐alkoxysilane 

precursors containing organo‐functional groups, like epoxy [84,92,93,95–107], methacrylic [36,108], 

or vinyl [15] that can be polymerized further increasing the coatingʹs density, and thus improving its 

barrier effect [84]. This shows better anticorrosive properties in comparison to hybrid sol‐gel coatings 

based on non‐functional organosilanes. Some other functional groups could be present, like phenyl 

[68,109], which  is used  to  increase coatings hydrophobicity or phosphonate  [55,110,111], which  is 

used to improve bonding to the substrate. Therefore, the different organic groups are used to achieve 

tailored properties of the sol‐gel coatings including compatibility with organic paints systems. 

Figure 6. Main end-uses for aluminum products in Europe (data from 2013). Adapted from [82].

Sol-gel coatings have been extensively studied as a promising environmental friendly alternativefor replacement of hexavalent chromium-based surface treatments of aluminium alloys [58]. This isnot an easy task since these conversion coatings are very effective in promoting both adhesion andcorrosion protection due to the active corrosion-inhibiting effect of Cr(VI) ions present in the film,besides the physical barrier induced by the film itself.

Solely inorganic sol-gel coating systems are porous and require densification with high curingtemperatures (above 300 ˝C) which can induce sensitisation as well as the loss of mechanical propertiesof the aluminium alloy substrate [83], making them unsuitable for corrosion protection of these alloys.However, hybrid sol-gel coatings, due to the presence of the organic component, which increases flexibilityand density, require much lower curing temperatures (below 150 ˝C) to achieve the desired barrierproperties making them more interesting and suitable for aluminium corrosion protection (Figure 7).Additionally, they have the ability to incorporate additives, such as inhibitors and nanoparticles, etc.,which further enhance corrosion and mechanical resistance of these types of coatings [55,84].

In 2001, Parkhill et al. [85] prepared epoxy-silica sol-gel films on 2014-T3 aluminium alloy.These results showed excellent barrier properties and enhanced corrosion resistance in salt spraytest in comparison to the chromate conversion pre-treatment conventionally applied to aircraft alloys.From this point forward, several types of organically-modified sol-gel coatings have been studied forthe corrosion protection of aluminium alloys [86–94]. Most of them are based on organo-alkoxysilaneprecursors containing organo-functional groups, like epoxy [84,92,93,95–107], methacrylic [36,108],or vinyl [15] that can be polymerized further increasing the coating's density, and thus improvingits barrier effect [84]. This shows better anticorrosive properties in comparison to hybrid sol-gelcoatings based on non-functional organosilanes. Some other functional groups could be present, likephenyl [68,109], which is used to increase coatings hydrophobicity or phosphonate [55,110,111], whichis used to improve bonding to the substrate. Therefore, the different organic groups are used to achievetailored properties of the sol-gel coatings including compatibility with organic paints systems.

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Figure  7.  Hybrid  sol‐gel  coating  synthesis  and  deposition  steps  in  the  laboratory:  (a)  separate 

hydrolysis of precursors (GPTMS and ZrTPO); (b) mixing sols; (c) dipping; (d) curing of aluminium 

substrates; and (e) visual aspect of sol‐gel coated samples. 

The amino groups have the ability to react with the epoxy groups and provide compatibility 

with organic polymer paint systems. Hence, the inclusion of amine‐derived cross‐linking agents in 

the  sol‐gel  synthesis  of  epoxy‐silica  based  hybrid  coatings  allows  the  promotion  of  the  organic 

network formation at low temperature [112], with inherent energy savings. Moreover, the process is 

essentially water‐based which is also environmentally benign. However, the amino‐cured hybrid sol‐

gel coatings developed at room temperature show reduced barrier properties in water with time due 

to low hydrolytic stability [113,114]. The corrosion behaviour of low temperature amine‐cured epoxy‐

silane sol‐gel coatings has been studied by Vreugdenhil et al. [115] and by Davis et al. [112] using 

diethylenetriamine  (DETA).  DETA  is  one  of  the most  common  epoxy  crosslinkers  resulting  in 

obtaining a dense protective coating able to provide aluminum corrosion protection. Khramov et al. 

[116] also studied epoxy‐silane sol‐gel coatings crosslinked with amino‐silanes and found significant 

improvement in these coating’s corrosion performance in comparison to those of DETA crosslinked 

ones. The amino‐silanes show the advantage of contributing also to the inorganic network formation. 

In addition to amino‐silanes, other amines have been studied as alternative to DETA like di‐amines 

of longer carbon chain [97] or branched amines [117,118], the resultant coatings showed improved 

corrosion protection properties  compared  to  those derived  from  formulations  containing DETA. 

Without  the  addition  of  amine  or  other  crosslinking  agents,  epoxy‐silane‐based  sol‐gel  coatings 

require thermal curing at temperatures above 100 °C to complete organic network formation. The 

introduction  of  organic  components  in  sol‐gel  films  results  in  thicker  crack‐free  coatings  with 

improved corrosion protection properties. However, it also decreases wear resistance and mechanical 

properties, making them prone to physical damage. To overcome this drawback several strategies 

have been studied, such as the addition of inorganic nanoparticles of silica, aluminium oxide, and 

other oxides to the sols [83,84,113,119], or the formation of nanostructured films or particles achieved 

by in situ synthesis during the sol‐gel process through controlled hydrolysis/condensation of sol‐gel 

precursors  [84,98,116,120–122]. The  latter process has  the advantage of providing chemical bonds 

between a hybrid matrix and nanoparticles. Examples of such coatings are those obtained by Donley 

et al. [116] through the self‐assembled nanophase particle process using tetramethoxysilane (TMOS) 

and GPTMS. Furthermore, hybrid coatings with incorporated zirconia nanoparticles prepared first 

by Zheludkevich et al. [121] from GPTMS, TEOS, and ZrTPO, and later by Fontinha [123] with the 

same  precursors  (excepting  TEOS)  (Figure  8),  and  the  epoxy‐zirconia‐silica‐based  sol‐gel  film 

prepared by Lamaka et al. [122] from GPTMS, ZrTPO, and titanium(IV) iso‐propoxide, (containing a 

nanostructured  porous  TiO2  interlayer)  also  revealed  that  the  incorporation  of  nanostructured 

Figure 7. Hybrid sol-gel coating synthesis and deposition steps in the laboratory: (a) separate hydrolysisof precursors (GPTMS and ZrTPO); (b) mixing sols; (c) dipping; (d) curing of aluminium substrates;and (e) visual aspect of sol-gel coated samples.

The amino groups have the ability to react with the epoxy groups and provide compatibilitywith organic polymer paint systems. Hence, the inclusion of amine-derived cross-linking agentsin the sol-gel synthesis of epoxy-silica based hybrid coatings allows the promotion of the organicnetwork formation at low temperature [112], with inherent energy savings. Moreover, the process isessentially water-based which is also environmentally benign. However, the amino-cured hybridsol-gel coatings developed at room temperature show reduced barrier properties in water with timedue to low hydrolytic stability [113,114]. The corrosion behaviour of low temperature amine-curedepoxy-silane sol-gel coatings has been studied by Vreugdenhil et al. [115] and by Davis et al. [112] usingdiethylenetriamine (DETA). DETA is one of the most common epoxy crosslinkers resulting in obtaininga dense protective coating able to provide aluminum corrosion protection. Khramov et al. [116]also studied epoxy-silane sol-gel coatings crosslinked with amino-silanes and found significantimprovement in these coating’s corrosion performance in comparison to those of DETA crosslinkedones. The amino-silanes show the advantage of contributing also to the inorganic network formation.In addition to amino-silanes, other amines have been studied as alternative to DETA like di-aminesof longer carbon chain [97] or branched amines [117,118], the resultant coatings showed improvedcorrosion protection properties compared to those derived from formulations containing DETA.Without the addition of amine or other crosslinking agents, epoxy-silane-based sol-gel coatingsrequire thermal curing at temperatures above 100 ˝C to complete organic network formation. The introductionof organic components in sol-gel films results in thicker crack-free coatings with improved corrosionprotection properties. However, it also decreases wear resistance and mechanical properties,making them prone to physical damage. To overcome this drawback several strategies have beenstudied, such as the addition of inorganic nanoparticles of silica, aluminium oxide, and otheroxides to the sols [83,84,113,119], or the formation of nanostructured films or particles achieved byin situ synthesis during the sol-gel process through controlled hydrolysis/condensation of sol-gelprecursors [84,98,116,120–122]. The latter process has the advantage of providing chemical bondsbetween a hybrid matrix and nanoparticles. Examples of such coatings are those obtained byDonley et al. [116] through the self-assembled nanophase particle process using tetramethoxysilane(TMOS) and GPTMS. Furthermore, hybrid coatings with incorporated zirconia nanoparticles preparedfirst by Zheludkevich et al. [121] from GPTMS, TEOS, and ZrTPO, and later by Fontinha [123] withthe same precursors (excepting TEOS) (Figure 8), and the epoxy-zirconia-silica-based sol-gel film

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prepared by Lamaka et al. [122] from GPTMS, ZrTPO, and titanium(IV) iso-propoxide, (containinga nanostructured porous TiO2 interlayer) also revealed that the incorporation of nanostructuredcomponents into the hybrid sol-gel films leads to the improvement of their barrier and mechanicalproperties. Recently, Rodic et al. reported the use of ZrTPO and methacrylic acid (MAA), followingtheir combination with sol synthesized from TEOS and 3-methacryloxypropyl trimethoxysilane.The various molar contents of ZrTPO of MAA were studied and the coatings offered high corrosionprotection to aluminum substrates under simulated aircraft conditions [108,124].

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components into the hybrid sol‐gel films leads to the improvement of their barrier and mechanical 

properties. Recently, Rodič et al. reported the use of ZrTPO and methacrylic acid (MAA), following 

their  combination with  sol  synthesized  from TEOS and 3‐methacryloxypropyl  trimethoxysilane. The 

various molar  contents of ZrTPO of MAA were  studied and  the  coatings offered high  corrosion 

protection to aluminum substrates under simulated aircraft conditions [124,108]. 

Additionally, these nanostructured components can be used to  introduce corrosion  inhibitors 

within the sol‐gel structure. 

 

Figure  8. AFM  image  of  an  epoxy‐zirconia‐silica hybrid  sol‐gel  coating  surface  topography with 

nanosized particles incorporated in the matrix [123]. 

Doping the hybrid sol‐gel coatings with corrosion inhibitors in order to reproduce a corrosion 

self‐healing  ability  of  chromate  conversion  layers  is  another  way  to  improve  sol‐gel  coatings’ 

corrosion performance aiming at the replacement of the toxic Cr(VI) surface treatments. Some of the 

most effective and environmentally friendly inhibitors are derived from cerium salts [119,125–127]. 

Good  results  have  also  been  reported with  aluminum  polyphosphate  [128].  Organic  inhibitors 

derived from  thiazole  [103,107,122,129] have also shown quite good results. However,  it has been 

reported that the presence of inhibitors within the sol‐gel coating, namely, when freely dispersed in 

the hybrid matrix, above a certain limit, has a detrimental effect in the long‐term chemical stability 

and barrier properties of the coatings [43,44,121]. Consequently, several research efforts in this field 

have been focused on the development of micro‐ and nanocontainers compatible with hybrid sol‐gel 

coatings that can be used for the controlled storage and release of the corrosion inhibitor (Figure 9) 

[39,40,130–141].  Some  examples  of  this  strategy  include  the  incorporation  of  cerium  salts  in 

nanostructured hybrid sol‐gel coating’s components synthesized in situ like the above mentioned, or 

the complexation of organic inhibitors with cyclodextrins [142]. 

 

Figure  9. Generic  scheme  showing  the  corrosion  healing mechanism  of  a  hybrid  sol‐gel  coating 

containing inhibitor loaded nanoparticles [123]. 

Other approaches to solve the problems related to long‐term storage of inhibitors involve their 

incorporation  in  “smart”  micro‐  and  nanocontainers  embedded  in  the  hybrid  matrix.  Such  as 

Figure 8. AFM image of an epoxy-zirconia-silica hybrid sol-gel coating surface topography withnanosized particles incorporated in the matrix [123].

Additionally, these nanostructured components can be used to introduce corrosion inhibitorswithin the sol-gel structure.

Doping the hybrid sol-gel coatings with corrosion inhibitors in order to reproduce a corrosionself-healing ability of chromate conversion layers is another way to improve sol-gel coatings’corrosion performance aiming at the replacement of the toxic Cr(VI) surface treatments. Some of themost effective and environmentally friendly inhibitors are derived from cerium salts [119,125–127].Good results have also been reported with aluminum polyphosphate [128]. Organic inhibitors derivedfrom thiazole [103,107,122,129] have also shown quite good results. However, it has been reportedthat the presence of inhibitors within the sol-gel coating, namely, when freely dispersed in the hybridmatrix, above a certain limit, has a detrimental effect in the long-term chemical stability and barrierproperties of the coatings [43,44,121]. Consequently, several research efforts in this field have beenfocused on the development of micro- and nanocontainers compatible with hybrid sol-gel coatings thatcan be used for the controlled storage and release of the corrosion inhibitor (Figure 9) [39,40,130–141].Some examples of this strategy include the incorporation of cerium salts in nanostructured hybridsol-gel coating’s components synthesized in situ like the above mentioned, or the complexation oforganic inhibitors with cyclodextrins [142].

Coatings 2016, 6, 12  10 of 19 

components into the hybrid sol‐gel films leads to the improvement of their barrier and mechanical 

properties. Recently, Rodič et al. reported the use of ZrTPO and methacrylic acid (MAA), following 

their  combination with  sol  synthesized  from TEOS and 3‐methacryloxypropyl  trimethoxysilane. The 

various molar  contents of ZrTPO of MAA were  studied and  the  coatings offered high  corrosion 

protection to aluminum substrates under simulated aircraft conditions [124,108]. 

Additionally, these nanostructured components can be used to  introduce corrosion  inhibitors 

within the sol‐gel structure. 

 

Figure  8. AFM  image  of  an  epoxy‐zirconia‐silica hybrid  sol‐gel  coating  surface  topography with 

nanosized particles incorporated in the matrix [123]. 

Doping the hybrid sol‐gel coatings with corrosion inhibitors in order to reproduce a corrosion 

self‐healing  ability  of  chromate  conversion  layers  is  another  way  to  improve  sol‐gel  coatings’ 

corrosion performance aiming at the replacement of the toxic Cr(VI) surface treatments. Some of the 

most effective and environmentally friendly inhibitors are derived from cerium salts [119,125–127]. 

Good  results  have  also  been  reported with  aluminum  polyphosphate  [128].  Organic  inhibitors 

derived from  thiazole  [103,107,122,129] have also shown quite good results. However,  it has been 

reported that the presence of inhibitors within the sol‐gel coating, namely, when freely dispersed in 

the hybrid matrix, above a certain limit, has a detrimental effect in the long‐term chemical stability 

and barrier properties of the coatings [43,44,121]. Consequently, several research efforts in this field 

have been focused on the development of micro‐ and nanocontainers compatible with hybrid sol‐gel 

coatings that can be used for the controlled storage and release of the corrosion inhibitor (Figure 9) 

[39,40,130–141].  Some  examples  of  this  strategy  include  the  incorporation  of  cerium  salts  in 

nanostructured hybrid sol‐gel coating’s components synthesized in situ like the above mentioned, or 

the complexation of organic inhibitors with cyclodextrins [142]. 

 

Figure  9. Generic  scheme  showing  the  corrosion  healing mechanism  of  a  hybrid  sol‐gel  coating 

containing inhibitor loaded nanoparticles [123]. 

Other approaches to solve the problems related to long‐term storage of inhibitors involve their 

incorporation  in  “smart”  micro‐  and  nanocontainers  embedded  in  the  hybrid  matrix.  Such  as 

Figure 9. Generic scheme showing the corrosion healing mechanism of a hybrid sol-gel coatingcontaining inhibitor loaded nanoparticles [123].

Other approaches to solve the problems related to long-term storage of inhibitors involvetheir incorporation in “smart” micro- and nanocontainers embedded in the hybrid matrix. Such as

Coatings 2016, 6, 12 11 of 19

functionalization of silica nanoparticles loaded with benzotriazole by a layer-by-layer (LbL) depositiontechnique coated with poly(ethyleneimine/poly(styrene sulfonate) (PEI/PSS) polyelectrolyte layersthat respond to pH change [143,144]. Similarly, natural halloysite nanotubes coated with polyelectrolyteshells by LbL were used to host the inhibitor 2-mercaptobenzothiazole [39]. In both cases a significantincrease in corrosion resistance of the doped hybrid coatings was achieved in comparison to the undopedones. Another type of “smart” reservoirs for corrosion inhibitors are those based on ion-exchange,like the layered double hydroxides (LDHs). LDHs (also known as anionic clays or hydrotalcite-likecompounds) consist of stacks of mixed-metal hydroxides, positively-charged, stabilized by anionsand solvent molecules located between the positive layers. The release of species from LDHs isbased on an anion-exchange mechanism. From the corrosion protection point of view, LDHs havea double role: (i) release of anionic corrosion inhibitors and (ii) entrapment of aggressive anionicspecies such as chlorides. Furthermore, the release of inhibitors can also be indirectly triggeredby pH changes: at very high pH the inhibitors can be exchanged with hydroxyl anions, whereasat very low pH the LDHs dissolve releasing the entrapped inhibitors [39]. Álvarez et al. [102]incorporated different concentrations of hydrotalcite compounds into hybrid ZrTPO-GPTMS sol-gelcoatings, the doped films showed increased adherence and corrosion resistance. In two other studies,concerning the incorporation of stimuli-response particles into hybrid epoxy-zirconia-silica sol-gelcoatings, calcinated red mud particles were used showing inhibitive action, and modified zeoliteswere used to host cerium (III) compounds, improving the protective performance of the hybridcoatings applied on AA 2014-T3 alloy [101,145]. Another strategy to incorporate and stabilize inhibitorspecies within hybrid sol-gel coatings is the use of multi-layered coatings. Rosero-Navarro et al. [146]developed an active sol-gel coating by sandwiching a Ce(III)-doped layer between two hybrid undopedlayers synthesized from TEOS, MAPTS, ethylene glycol dimethacrylate, and glycidyl methacrylate.The corrosion performance between a three-layered inhibiting system was compared with a passiveundoped bi-layered system. The former revealed enhanced corrosion properties when comparedto the latter. According to the authors, this coating system was developed aiming to be employedeither as a complete corrosion protection system without a top coat or as primer for successiveapplication of water-based top-coats. Recently, Ferreira et al. [147] reported the synthesis of polyureamicrocapsules loaded with corrosion inhibitor 2-mercaptobenzothiazole (MBT) for corrosion protectionof 2024 aluminum alloy. The microcapsules were prepared by interfacial polycondensation and addedto a hybrid sol-gel coating. The authors showed that results obtained indicate that capsules loadedwith MBT do not affect the barrier properties of the coatings, and enhanced the adhesion to the metallicsubstrate. Baraka et al. [106] extracted collagen powder and dissolved it within a hybrid silica sol-gel toproduce an environmental friendly (green) coating. The authors reported that the collagen improvedthe corrosion resistance of the coated AA2024 and did not compromise the mechanical properties ofthe coating. For aluminium substrates, more precursors have been tested than for steel alloys, howeverthe reported data shows that the most used precursors are TEOS, GPTMS, TMOS, and ZrTPO which isaccording to the tendency found until 2013 [58].

The enormous number of studies performed with hybrid sol-gel coatings showed the effectiveability of these coatings to protect aluminum alloys from corrosion. However, very few involved testingtheir performance with industrial organic paints [113,117,123,148] in order to assess the compatibility ofboth systems and with the industrial requirements (Figure 10) that generally are only partially fulfilled.

The introduction of organosilane based hybrid coatings on an industrial scale is still in an earlystage of development due to economic reasons and to the limitations imposed for the industrialuse of these products related to the long-term stability of the sols. Additionally, some of the higherperformaning solutions involve complex formulations and conditions of preparation that might bedifficult to achieve in large-scale industrial applications. Future demands will involve the developmentof integrated solutions where hybrid coatings are mixed with conventional components to createcoatings that will combine functions of conversion coatings, primer, and topcoat, and of course, withself-healing ability.

Coatings 2016, 6, 12 12 of 19Coatings 2016, 6, 12  12 of 19 

 

Figure 10. (a) Polyester powder coated aluminum alloy 6063 sheets pre‐treated with a hybrid sol‐gel 

doped with Ce(III)  inhibitor after 1000 hours salt spray  (NSS)  test  (ISO 9227);  (b) Neither signs of 

aluminum corrosion outside the scribe, nor signs of blistering or infiltration over the scribe length, 

were visible on the coating [123]. 

3. Future Hybrid Coating Systems and Approaches 

It  is  undeniable  that  the  future  applications  and  research will  be  focused  primarily  on  the 

investigation of more environmental  friendly precursors/reagents or  industrial colloid particles  to 

replace the traditional precursors in the formation of sol‐gel protective coatings without changing the 

general properties. Combined systems will be also developed in order to achieve the best protection 

possible  against  corrosion,  such  as  metal‐rich  sol‐gel  coatings,  containing  zinc  or  magnesium 

particles,  or  other  inhibitor  particles, which  combine  the  barrier  properties  of  sol‐gel  layer  and 

cathodic protection of sacrificial particles; multilayer systems, such as the one proposed in Figure 11 

in which a pre‐emptive behavior is considered. This system combines different sol‐gel layers with 

different functions, which is a very promising area to explore. 

 

Figure 11. A possible schematic illustration of a three‐layer system based on hybrid sol‐gel coatings 

and the  illustration of how the pre‐emptive healing system works. Water and electrolyte diffusion 

through the sol‐gel topcoat can generate corrosion reactions at the metal‐oxide interface. Pre‐emptive 

healing could occur through the trapping of water and corrosives and/or the release of inhibitors from 

the hybrid sol‐gel, which targets the corrosion reactions and shuts them down. Self‐healing agents 

also heal the top‐coat from the effects of weathering and/or other damage. 

The properties of sol‐gel coatings, such as delamination, crackability, and adhesion, are crucial 

factors to ensure the quality of coatings for corrosion mitigation. However, a systematic theory and 

procedure on how  to evaluate  the viability, efficiency and stability of  these coatings has not been 

implemented  yet.  Nevertheless,  to  guarantee  the  achievement  of  efficient  sol‐gel  coatings  for 

corrosion mitigation,  these procedures must be  considered and applied. Hybrid  sol‐gel materials 

synthesized and found in the literature demonstrated market viability either due to unique properties 

Figure 10. (a) Polyester powder coated aluminum alloy 6063 sheets pre-treated with a hybrid sol-geldoped with Ce(III) inhibitor after 1000 hours salt spray (NSS) test (ISO 9227); (b) Neither signs ofaluminum corrosion outside the scribe, nor signs of blistering or infiltration over the scribe length,were visible on the coating [123].

3. Future Hybrid Coating Systems and Approaches

It is undeniable that the future applications and research will be focused primarily on theinvestigation of more environmental friendly precursors/reagents or industrial colloid particles toreplace the traditional precursors in the formation of sol-gel protective coatings without changing thegeneral properties. Combined systems will be also developed in order to achieve the best protectionpossible against corrosion, such as metal-rich sol-gel coatings, containing zinc or magnesium particles,or other inhibitor particles, which combine the barrier properties of sol-gel layer and cathodic protectionof sacrificial particles; multilayer systems, such as the one proposed in Figure 11 in which a pre-emptivebehavior is considered. This system combines different sol-gel layers with different functions, which isa very promising area to explore.

Coatings 2016, 6, 12  12 of 19 

 

Figure 10. (a) Polyester powder coated aluminum alloy 6063 sheets pre‐treated with a hybrid sol‐gel 

doped with Ce(III)  inhibitor after 1000 hours salt spray  (NSS)  test  (ISO 9227);  (b) Neither signs of 

aluminum corrosion outside the scribe, nor signs of blistering or infiltration over the scribe length, 

were visible on the coating [123]. 

3. Future Hybrid Coating Systems and Approaches 

It  is  undeniable  that  the  future  applications  and  research will  be  focused  primarily  on  the 

investigation of more environmental  friendly precursors/reagents or  industrial colloid particles  to 

replace the traditional precursors in the formation of sol‐gel protective coatings without changing the 

general properties. Combined systems will be also developed in order to achieve the best protection 

possible  against  corrosion,  such  as  metal‐rich  sol‐gel  coatings,  containing  zinc  or  magnesium 

particles,  or  other  inhibitor  particles, which  combine  the  barrier  properties  of  sol‐gel  layer  and 

cathodic protection of sacrificial particles; multilayer systems, such as the one proposed in Figure 11 

in which a pre‐emptive behavior is considered. This system combines different sol‐gel layers with 

different functions, which is a very promising area to explore. 

 

Figure 11. A possible schematic illustration of a three‐layer system based on hybrid sol‐gel coatings 

and the  illustration of how the pre‐emptive healing system works. Water and electrolyte diffusion 

through the sol‐gel topcoat can generate corrosion reactions at the metal‐oxide interface. Pre‐emptive 

healing could occur through the trapping of water and corrosives and/or the release of inhibitors from 

the hybrid sol‐gel, which targets the corrosion reactions and shuts them down. Self‐healing agents 

also heal the top‐coat from the effects of weathering and/or other damage. 

The properties of sol‐gel coatings, such as delamination, crackability, and adhesion, are crucial 

factors to ensure the quality of coatings for corrosion mitigation. However, a systematic theory and 

procedure on how  to evaluate  the viability, efficiency and stability of  these coatings has not been 

implemented  yet.  Nevertheless,  to  guarantee  the  achievement  of  efficient  sol‐gel  coatings  for 

corrosion mitigation,  these procedures must be  considered and applied. Hybrid  sol‐gel materials 

synthesized and found in the literature demonstrated market viability either due to unique properties 

Figure 11. A possible schematic illustration of a three-layer system based on hybrid sol-gel coatings andthe illustration of how the pre-emptive healing system works. Water and electrolyte diffusion throughthe sol-gel topcoat can generate corrosion reactions at the metal-oxide interface. Pre-emptive healingcould occur through the trapping of water and corrosives and/or the release of inhibitors from thehybrid sol-gel, which targets the corrosion reactions and shuts them down. Self-healing agents alsoheal the top-coat from the effects of weathering and/or other damage.

The properties of sol-gel coatings, such as delamination, crackability, and adhesion, are crucialfactors to ensure the quality of coatings for corrosion mitigation. However, a systematic theoryand procedure on how to evaluate the viability, efficiency and stability of these coatings has notbeen implemented yet. Nevertheless, to guarantee the achievement of efficient sol-gel coatings forcorrosion mitigation, these procedures must be considered and applied. Hybrid sol-gel materials

Coatings 2016, 6, 12 13 of 19

synthesized and found in the literature demonstrated market viability either due to unique propertiesthat allow new end-use applications or show a significantly enhanced cost/performance relationshipcompared to the available materials in the marketplace. However, it must be emphasized that thereare many anti-corrosive epoxy coatings commercially available and the continuous research forcontemporaneous hybrid polymers has been gaining market niches, leading us to believe that theseare the directions for future success. On the other hand, it was also shown that some hybrid materialsalready exist with significant commercial potential; however, these have not yet reached commercialstatus. Like all innovative materials, these are facing many difficulties and challenges for large-scaleindustrial production since a number of factors, including capital investment, ease of manufacturing,coating performances, and environmental issues need to be considered when developing a coatingprocess for industrial application. Therefore, future trends for hybrid sol-gel coatings are predicted tobe towards low cost, pollution-free, ease of synthesis, and effective in corrosion control. These obstaclesand difficulties should be easily overcome by establishing a direct connection between the industry,the users/custumers and the scientists, creating a synergy in which everyone wins.

Author Contributions: Rita B. Figueira contributed to the definition of the review structure based on availableliterature survey, to writing, to integration of the different parts and to conclusions and was also the editor ofthe manuscript, main reviewer of content and formatting; Isabel R. Fontinha contributed to the definition ofthe review structure based on available literature survey, to writing, to integration of the different parts and toconclusions. Carlos J. R. Silva and Elsa V. Pereira contributed to the definition of the review structure based onavailable literature survey.

Conflicts of Interest: The authors declare no conflict of interest.

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